3.' 



CNDULATORY FORCES. LIGHT. 



[l-OULUlSCOFI 



polarised light in ghua, aci<U, nlU, crystal*, <tc. 

 adapted for nit-lit or iUy oouninU of a polarising mirrur, 

 a glan-ctand to give the proper angle, and an analysing 

 tube ; all of the uiont simple and cheap fonu, with all 

 tin- propertiM of the moat expensive apparatus. 

 fg.1t. 



ectiont for vtiny Bettalfi Polariting Appurahu. 

 Hold tin' red ! ""', in <>iio hand; pull the rod 



tongue, B, with the other that will place the mirror and 

 the glass at the proper angle. Place the black mirror in 

 the DOX, A, with the reflecting port upwards ; and the 

 glads for giving the angle on the mirror, as in the 

 drawing. Place the object on glass, C, and fix the 

 ground glass to the light. Place the apparatus on a table 

 by the window, for day ; for night, a light to reflect about 

 half-way up the ground glass should be used. Hold the 

 analysing box, D, to the eye, turn it gently round, and sit 

 about two or three feet from the apparatus ; look at the 

 object, and so obtain the phenomena of polarised light. 



By turning the analysing tube round,, the primary 

 and complementary colours will be afforded. 



The analysing plates can be taken out of the tube to 

 clean them ; but be particular in replacing tliem in their 

 r position. 



Having tried the above-described arrangement, we 

 can unhesitatingly state that it is a most useful instru- 

 ment It has, also, the recommendation of being as 

 cheap as it is effective. 



The most commonly used fonu of polariscopo is that 

 represented in the annexed figure. In this apparatus, 

 polarisation takes place by reflection, and the analyser 

 is also a reflector. It is an extremely convenient ar- 

 rangement for the purpose of showing, at alternate 

 angles of 90, the phenomena of complementary colours. 

 We observe, that at the lower part of this polariscope 

 7*3), a square frame, containing a few glass plates 

 I:ist of which has been well blackened), is fixed. On 

 this the ray of common light is allowed to fall at an 

 ;ui,'le of 33. Being reflected from the surface, the 

 polarised ray reaches the analyser, which consists, also, 

 of a number of glass plates (the bottom one being black- 

 ened). The object to be viewed is placed on an inter- 

 mediate stage, and the eye of the observer sees the 

 effect* of polarised light by means of the top mirror. 



Tn Fig. 76, a represents the polarising plate ; 6 is the 

 analysing plate; c. is the stage <>n which the object to be 

 iiiifl !> polarised light is placed. The frame hold- 

 ing the analysing plate, 6, is movable, so that it may be 

 turned round, UM bo so arranged at any angle with the 



polarising plate. The angles are nn-asinvd by a gra- 

 duated run, as observed in the engraving; d show 

 direction of a ray of light incident on the polarising plate, 

 a, which, passing to 6, is reflected to e. This is the 

 position in which the eye of the observer is placed, lioth 

 the analysing and jHilarising plates are placed on pivots 

 similar to those employed in common looking-glasses. 

 This arrangement permits of their being placed at any 

 angle. In employing this instrument, the polarising 

 plate should receive a ray of light at an ancle of al>out 

 57; and the analysing plate should be similarly arranged, 

 and receive the ray from a, at an angle also of 57. A 

 considerable beam of polarised light may be thus ob- 

 tained. Wo may remark, that such an apparatus uiuy 

 easily be constructed by any < iKim-t or looking . 

 manufacturer, by following the detailed instructions wo 

 have here given. 



The arrangement of the polariser and analyser, in our 

 engraving, throws the two at such an angle as will aili 1 

 the ordinary phenomena of polarised light. By turning 

 the analyser to an angle of 90 with the polariser, the 

 complementary colours to those seen in any object by 

 the represented arrangement will be observed most 

 readily. We may remark, that 0, 180, and 300, are 

 the relative positions of the two plates for the reflection 

 of the polarised ray, and 90 and 270 for that of its 

 absorption or extinction. At these the complementary 

 colours become visible. We have hitherto confined our- 

 selves to the description of the simplest forms of polari- 

 scopes. Others of a more exact character, in which 

 tourmaline plates, Nicol's prism, &c., are employed, will 

 be described as we proceed. 



GENERAL PHENOMENA OF POLARISED 

 LIGHT. 



FBOM what we have stated in reference to the construc- 

 tion of polariscopes, our readers will have perceived, that 

 all the effects of polarised light depend on the peculiar 

 molecular construction of bodies. Indeed, one of the 

 most valuable uses, in a philosophic point of view, to 

 which the polariscope can be applied, is that of detecting 

 differences which exist in the arrangement of the atoms* 

 of which a mass is composed. We are enabled to deter- 

 mine, with a most astonishing precision, the charac- 

 teristics of two or more bodies having exactly the same 

 chemical constitution, and yet differing in their purely 

 physical character. An interesting illustration of this 

 fact is found in the case of sugar, and many kinds of 

 starch, which, whilst presenting the same results as to 

 quantity and proportion when subjected to the test of 

 chemical analysis, exhibit astonishing differences when 

 examined by means of the pohuiscope. 



To those who take a pleasure in studying natural 

 laws, and their special application, this fact must neces- 

 sarily be one of the most interesting kind. A iiuml>< -r 

 of bodies is taken, each member of which has the same 

 elements for its constitution ; and each of these may IHJ 

 found exactly in the same ratio in one as in the other ; 

 yet the whole group differs individually. Nothing can 

 seem more heterogeneous than sugar and starch ; and 

 yet, chemically, their constitution is the same. Then, 

 again, we have the diversity of character in the same 

 family of starches, aa potato, rice, arrowroot, tapioca, 

 Hour, <kc. ; each of which differs from its fellow in many 

 of its qualities ; such difference, however, almost entirely 

 escaping the keen eye of the practical chemist Again, 

 some bodies have so singular a relation to, or power 

 over, a ray of light passing through them, as not only 

 to alter its course, as in the ordinary case of single and 

 double refraction, but also to divide or split it, and so 

 to produce a series of phenomena of the most extraor- 

 dinary kind. Hence we perceive the effects of plane 

 and circular polarisation. 



\Vi- may here remark on the fact, that whilst the 



generally received idea invests the .|ii:iUtios of the taste, 



Hindi, itc., of bodies with a chemical rhar.u KT, the laws 



of polarisation tent! to enlarge our views m reference to 



Dec Introduction, p. 1. 



